1. Field of the Invention
The present invention relates to an optical disk device and an optical pickup and more particularly, to making discrimination among a plurality of kinds of optical disks.
2. Description of the Related Art
In JP-A-8-287588, the thickness of a cover layer is decided to discriminate the kind of an inserted optical disk by making the focus of a light beam approach the optical disk and measuring a time for the focus to reach an information recording layer from the surface of the optical disk.
An available optical pickup capable of correcting spherical aberration is constructed by having a beam expander disposed in an optical path of flux of light to add a predetermined spherical aberration, having a liquid crystal element for giving a predetermined spherical aberration or having a means for changing the distance between two objective lenses in combination in the direction of focusing to provide spherical aberrations. Presently, a technology has been known which uses an optical pickup constructed as above to intentionally generate a spherical aberration in order to cancel out a spherical aberration caused by an error in cover thickness or an error in spacing between information recording layers.